This invention was made in the course of or under a contract with the Energy Research and Development Administration. It relates to the art of nuclear fuel reprocessing and more particularly to the art of recovering cesium and palladium values from fission product waste solution.
In nuclear fuel reprocessing systems, spent fuel elements are periodically removed from the reactor and the fission products are separated from the fissionable uranium, plutonium and neptunium. Typically, the component containing the fission products leaves the first extractor as a nitric acid solution containing a variety of metal ions. As used herein the term "fission product waste solution" refers to a solution of the various metal ions which remain in solution after spent, irradiated nuclear fuel is dissolved in acid and the fissionable materials selectively extracted from the solution. This fission product waste solution contains several useful radioisotopes, such as .sup.137 Cs and .sup.90 Sr. In addition, several precious metals such as palladium, ruthenium and rhodium are present in recoverable quantities. It is therefore profitable that these elements be removed prior to long-term storage of the waste material. Furthermore, because of the long half-lives of .sup.137 Cs and .sup.90 Sr it is desirable that they be removed as much as possible from the waste materials prior to storage.
In most fuel reprocessing plants, for example the Midwest Fuel Recovery Plant in Morris, Illinois, fuel rods are removed from their assemblies, sheared to the desired length, and fed to a continuous leacher for dissolution with nitric acid. This leacher solution is separated from the metal hulls, etc. and is transferred to an extraction column where it is contacted with a solvent (typically tributyl phosphate in an organic diluent) which selectively extracts the U, Pu, and (if desired) Np values from the solution. The extractant solution is then treated according to one of several well-known processes to further separate and recover the fissionable material. Typical examples are the Purex process, the "25" process, and the Butex process, all more fully described in Chemical Processing of Reactor Fuels, Academic Press, New York and London (1961) which is herein incorporated by reference.
The aqueous nitric acid solution (raffinate) remaining in the extraction column contains the fission products. The fission product solution is typically withdrawn from the extractor and subjected to further treatment in preparation for long-term storage. Fission product waste solution has a very complex composition, consisting of various metal ions in several oxidation states as well as a variety of soluble complex ions.
The basis of this invention lies in the discovery that two valuable fission products may be recovered from fission product waste solution via the formation of a stable compound, Cs.sub.2 PdCl.sub.6, which precipitates when the solution is oxidized to convert Pd ions to the IV oxidation state and contacted with a soluble chloride or other source of chloride ions.